Activating method for cesium activated iii-v compound photocathode using rare gas bombardment

ABSTRACT

A method for activating a cesium-activated III-V compound photocathode is described. Prior to covering with the cesium, the cathode surface is subjected to intensive cleaning by heating for several hours at a maximum temperature of 300* C. and then the surface is bombarded with slow rare gas ions.

United States Patent Inventors Siegfried Garbe;

Gunter Heinrich August Frank, both of Aachen, Germany Appl. No. 806,192 Filed Mar. 11, 1969 Patented Dec. 28, 1971 Assignee U.S. Philips Corporation, New York, N.Y. Priority Mar. 15, 1968 Germany P 16 39 363.8

ACTIVATING METHOD FOR CESIUM ACTIVATED III-V COMPOUND PHOTOCATHODE USING RARE GAS BOMBARDMENT 5 Claims, 1 Drawing Fig.

U.S. Cl 316/12, 316/13, 316/26 Int. Cl H0lj 9/38 Field 01 Search 316/12, 13, 26, 3, 24

Photoemissive Materials" by Sommer pp. 42 47 and 61- 63 (publ. 1968 by John Wiley & Sons).

Primary Examiner-John F. Campbell Assistant Examiner--Richard Bernard Lazarus Attorney-Frank R. Trifari ABSTRACT: A method for activating a cesium-activated lIl-V compound photocathode is described. Prior to covering with the cesium, the cathode surface is subjected to intensive cleaning by heating for several hours at a maximum temperature of 300 C. and then the surface is bombarded with slow rare gas ions.

PATENTED 050281971 INVENTORS. SIEGFRIED GARBE symsn H A.FRANK ACTIVATING METHOD FOR CESIUM ACTIVATED Ill-V COMPOUND PHOTOCATHODE USING RARE GAS BOMBARDMENT The invention relates to a method of manufacturing an electric discharge tube, having a photocathode, the active constituent of which consists of a strongly p-conductive A E,- compound which is activated by an alkali metal or an alkaline earth metal.

An A,,,B,-compound is to be understood to mean herein normally an intermetallic compound of one of the elements (A,,,) of the third group of the periodic system, boron, aluminum, gallium, indium on the one hand, with an element B,- of the fifth group of the periodic system, nitrogen, phosphorus, arsenic, antimony on the other hand. Mixed crystals are also included.

Such photocathodes were described by J. .l. Scheer and J. van Laar in Solid State Communications" 3, 189-l93, l965. The A B cathode was formed from a monocrystal which was cleaved in vacuo.

The manufacture of a photocathode from crystals which are not cleaved in air, or from layers manufactured epitaxially or by vapour-deposition and sputtering, respectively, of such compounds often is not successful.

It could be established on the basis of various phenomena that an intensive cleaning of the surface to be used for photoemission is necessary, but that the cleaning methods, as they were used so far, dp not give full satisfaction.

In a method according to the invention of manufacturing an electric discharge tube having a photocathode the active constituent of which consists of a strongly p-conductive A E,- compound which is activated by an alkali metal or alkaline earth metal, the not yet activated compound in the tube is subjected to a heating process at a temperature of maximally 300 C. for a few hours, after which the surface of the compound is subjected to a bombardment by slow rare gas ions.

The temperature during the heating process is chosen to be so low because otherwise oxidation of the compound occurs by released residual gases. The temperature is also low enough to avoid out-diffusion of the doping substance from the compound.

During the bombardment with slow rate gas ions, various layers of the surface are removed, so that contaminations at the surface, for example, oxides are also removed. The energy of these ions is preferably halved during a second part of the treatment. The energies for argon are then maximally 100 and 50 ev., respectively. The current density of the ion bombardment may be of the order of 30 uA per sq. cm. The reduced energy produces a further removal without the formation of crystal defects.

The high yields of the known photocathodes are obtained by activation with, for example, monolayers of cesium. The danger exists, however, that after a rather short period of time the cesium equilibrium between the photocathode and the other surfaces in the .tube is disturbed, in which traces of residual gas may also play a part. In order to avoid this, the activation by means of cesium or a different activator may be continued, in a manner already proposed, until the photocurrent reduces again to approximately half. By admitting oxygen, the photoemission is increased again and the activation is repeated a few times in this manner. Often only 80 percent of the maximum yield is obtained.

According to the invention, the method is continued until the activation comprises at most 5 to monolayers, while after the said oxygen treatment, a fraction of a monolayer is supplied. Substantially the same yield as in a monolayer is obtained, while the emerging electrons are substantially monoenergetic in contrast with the electrons emitted by a monolayer.

The cathodes activated in this manner with various monolayers must be formed at temperatures of 150 to 170 C. for IS to 60 minutes before the fraction of the monolayer is provided. At the same time an improvement of crystal defects in the semiconductor compound is obtained.

In order that the invention may be readily carried into effect, one embodiment thereof will now be described in greater detail, by way of example, with reference to the accompanying drawing.

Reference numeral 1 in the drawing denotes a glass tube with exhaust tube 2 and light entrance window 3. The cathode 4 is a monocrystalline gallium arsenide plate the surface of which has (1 l0) orientation. 5 is a gallium arsenide layer, 10 microns thickness, provided epitaxially on the surface. The layer is doped with 3X10 atoms of zinc per ccm. Making it strongly P-type. 6 is the annular anode for the photocathode, 7 is a gridlike electrode and 8 is a platelike electrode. 9 is a cesium evaporation source. After providing all the electrodes in the tube, it is heated in chlorothene, for degreasing, at 60 C. for 2 minutes. The exhaust tube 2 is then connected to the pump. After evacuation the cathode with the whole tube is heated at 275 C. for 4 hours.

Pure argon is then admitted to the tube under a pressure of approximately 5X10 Torr. (Traces of oxygen are removed from the argon by a gettering substance). Between the electrodes 7 and 8 a discharge is ignited by closing switch 10 and a negative voltage of maximally volt is applied between the electrode 7 and the cathode 4 by closing switch 11 and hence an argon ion current of 30 ua./sq.cm. is drawn to the surface of the cathode 4 for 30 minutes producing the slow rare gas ion bombardment of the photocathode as previously described. The voltage is then reduced by rheostat 12 to 50 volts and current is drawn for another 15 minutes.

Then the ion bombardment is terminated, and after pumping away the argon, the cesium source 9 is energized by switch 13 and so much cesium is evaporated from the cesium evaporator 9 already degassed during the heating process until the cathode 4 has approximately a maximum photoemission towards the anode 6. This is measured during the cesium activation step in the usual manner by applying a voltage between the anode 6 and cathode 4 via switch 14 and measuring the current in the external circuit, which is the photocathodes photocurrent, while light is incident thereon from above. Then so much more cesium is evaporated onto the cathode surface 5 until the measured photocurrent is halved. Oxygen of approximately 5X10 Torr is then admitted to the tube 1 for such a period of time until the measured photocurrent again reaches a maximum. This cesium activation treatment and oxidation are repeated a few times until the photocurrent again very nearly reaches its first maximum. The cathode 4 is then heated at for 30 minutes after which cesium is again evaporated from the source 9 in a quantity which corresponds approximately to one-fourth to half of the first cesium addition. This corresponds to the monolayer fraction above described.

What is claimed is:

l. A method of activating a photocathode within an electric discharge tube, said photocathode comprising a surface of a strongly P-type conductivity compound of an element selected from the group consisting of boron, aluminum, gallium, and indium with an element selected from the group consisting of nitrogen, phosphorus, arsenic, and antimony, and mixed crystals thereof, covered with an activating material of an alkali metal or alkaline earth metal, comprising the steps of heating the unactivated compound within an evacuated tube at an elevated temperature not exceeding 300 C. for several hours, thereafter subjecting the surface of the unactivated compound to slow rare gas ion bombardment with ions whose energy is maximally 100 ev., and thereafter bringing the activating material into contact with the bombarded surface of the compound.

2. A method as set forth in claim 1 wherein the rare gas ion bombardment step comprises a first part using rare gas ions of a given energy, and a later second part using rare gas ions whose energy is approximately half of the said given energy.

3. A method as set forth in claim 2 wherein the ions are argon.

minutes, and thereafter the surface of the cathode is covered with a fraction of a monolayer of the activating material.

5. A method as set forth in claim I wherein the compound is gallium arsenide, and the activating material is cesium.

2;;2; UNITED STAT PATENT OFFICE ETTTTTMT h 3,630 ,5 87 Dated December 28, 1971 Patent No.

Inventor) Siegfried Garbe et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title Page, Col. 1, line 10, "P 16 39 363.8" should read Sied and sealed this 2nd day of 14931972.,

(SEAL) Attest:

EDWARD MTFLETGHHTR .05Rw ROBERT GOTTSGHALK Attesting Officer 7 Gomissioner of Patents 

2. A method as set forth in claim 1 wherein the rare gas ion bombardment step comprises a first part using rare gas ions of a given energy, and a later second part using rare gas ions whose energy is approximately half of the said given energy.
 3. A method as set forth in claim 2 wherein the ions are argon.
 4. A method as set forth in claim 1 wherein the step of bringing the activating material into contact with the bombarded surface is carried out as follows: the surface is first covered with activating material, thereafter the covered surface is subjected to an oxygen atmosphere, thereafter the cathode is heated at a temperature of 150*-170* C. for 15-60 minutes, and thereafter the surface of the cathode is covered with a fraction of a monolayer of the activating material.
 5. A method as set forth in claim 1 wherein the compound is gallium arsenide, and the activating material is cesium. 